Examples of reconfigurable semiconductor integrated circuits include an FPGA (field-programmable gate array). The FPGA includes reconfigurable logic cells and reconfigurable interconnections and allows a user to reconfigure the FPGA by programming the logic cells and interconnections using design tools. Consequently, the user can obtain a semiconductor integrated circuit provided with desired functions in his/her own hands. The FPGA is a general-purpose component and several types of FPGA which differ in logic cell scale, peripheral circuits, and the like are available according to the purpose.
The FPGA has two advantages. The first is that since there is no need to design semiconductor integrated circuits for users, the FPGA almost eliminates the need for design and development costs tailored to specific users and initial costs for masks and the like for manufacture of integrated circuits. The second is that since there is no need to newly develop and manufacture semiconductor integrated circuits, development periods can be reduced greatly.
However, since the FPGA is equipped with switches to configure logic cells and interconnections and is provided with logic cell regions not used by the user, chip size is several tens of times larger than that of a custom-designed integrated circuit. Consequently, manufacturing costs per chip, excluding initial costs, is very high. Also, the large chip size results in long wiring length, low operating speed, and high power consumption. Therefore, when the user needs a large number of integrated circuits or high-performance integrated circuits, custom-designed integrated circuits are suitable.
The area of the switches used to reconfigure logic cells and interconnections takes up more than half the FPGA chip area and greatly affects performance and manufacturing costs of the FPGA. The switches currently in use are a type known as an SRAM switch which is a combination of an SRAM (static random-access memory) and a pass transistor. The SRAM switch has the advantage of being able to be manufactured by the same method as logic cells.
Desirably the switches used for the FPGA have a small switch area and low ON resistance. The SRAM switch has a switch area of 120F2 (where F is a minimum processing dimension of an integrated circuit) and an ON resistance of approximately 1 k ohm.
An example of switches is disclosed in JP2006-339667A (hereinafter referred to as Patent Document 1). The switch disclosed in Patent Document 1 has achieved a switch area of 8F2 and an ON resistance of approximately 100 ohms. Therefore, if the switch can be used for the FPGA, the performance of the FPGA can be improved and the manufacturing costs of the FPGA can be reduced greatly.
An FPGA using the switches disclosed in Patent Document 1 can mount a larger number of logic cells than an FPGA using SRAM switches if it is assumed that the FPGAs have the same chip size. Therefore, an application that cannot be mapped to an FPGA which uses SRAM switches due to an upper limit to the number of logic cells will be able to run on an FPGA which uses the switches disclosed in Patent Document 1.
Now, operation of the switch disclosed in Patent Document 1 will be described briefly. The switch is a three-position switch equipped with three electrodes. The switch does switching from the OFF state to ON state by forming a current path in an ion-conducting layer between the first and second electrodes. The current path connecting the two electrodes is generated and extinguished by an electrochemical reaction caused by precipitation and dissolution of copper ions, and the electrochemical reaction is controlled by the third electrode. Incidentally, the ion-conducting layer is also known as an ionic conductor or solid-electrolytic layer, but the term “ion-conducting layer” will be used hereinafter.